Using an object as a clipping mask for another object
With a scene set up like your image, a wedge shape and the curve shape with an array modifier; I created the mask with the compositor.

I have a fully transparent material on the wedge, and the material pass set to 1 (object index pass would work here as well.) Do note that the wedge's top face has to be above the curve shape for the IndexOB or the IndexMA to give a usable mask.

In the compositor use the material, or object index pass as a mask by multiplying (a Mix node) it with the image's alpha (because the material index pass is set to one there is no need for a Id Mask node). Then take the black and white masked image from the mix node and use it as the image's alpha with a Set Alpha node.


Here is the result after using the IndexOB as a mask.